CO2 equivalent / kWh produced electricity

Electricity production causes emissions, but different energy sources and technologies have a different impact on greenhouse gases. This article shows the caused CO2 equivalent of emissions per kWh produced electricity for the most common energy sources.

Generally, everything that human beings create, come at the cost of emission production. Looking at the life cycle costs, though, reveals the differences. This is no different for electricity production. Several studies investigated this topic and the results vary heavily. Therefore, all findings need to be understood as estimations which are constantly changing with new technologies, adopted supply chains, and even changed geographic situations.

We base our ranking on two studies: A research published in Nature Energy1, as explained in CarbonBrief, and findings by the IPCC as published by EnergyForHumanity.

Renewables are cleaner

Nature energy’s findings show, that most renewable energy sources are much cleaner than fossil fuels. This might not come as a surprise. What maybe does come as a surprise is, that – as already in the death rate study – nuclear power wins. The researchers found that nuclear plants come with an emissions footprint of 4 grammes of CO2 equivalent (gCO2e/kWh). The first place is shared with wind energy, which also accounts 4 grammes of CO2 equivalent over its lifetime.

Marginally behind wind and nuclear power, comes solar energy with 6 gCO2e/kWh. This includes various types of solar energy and it is to be expected that new technologies and more efficient solar panels will help to continuously decrease this number.

Hydropower and bioenergy trapped in the middle

Somewhere between the aforementioned renewables and fossil fuels lie hydropower and bioenergy. While those sources are renewable, they are not necessarily clean or green, as the study notes. Hydropower (97g CO2 equivalent) and bioenergy (98g) have highly variable lifecycle emissions. For hydro dams drawback are issues due to the rotting organic matter flooded by the dam. This means certain sites should be avoided, in particular, shallow dams in warm regions with large variations in water level.

Bioenergy is critical as its greenhouse gas impact depends on how the biomass is sourced, where it was cultivated, as well as on many more aspects.

Fossil fuels with the biggest impact

Unfortunately, this is where the (relatively) good numbers end. The study finds that gas CCS (the most modern form of converting gas into electricity) already produces 78 grammes of CO2 equivalent. Coal CCS (109g) is still seen as the dirtiest of all energy sources despite attempts of making it “cleaner” or “greener”. Now, CCS, standing for Carbon Capture and Storage, is a group of technologies that can remove almost 100% of the carbon dioxide emitted from large-scale point sources of carbon such as energy-intensive industries and fossil fuel power. While the study in Nature Energy acknowledges this, it also finds that upstream emissions during mining of coal or extraction of gas continue and that CCS only captures 90% of power plants CO2. Higher capture rates could solve the problem but are very costly and would not eliminate upstream emissions, which alone results in an equivalent to 23-42gCO2e/kWh.

Different study, different results

The second study by the IPCC is rather older and dates back to 2011. This has implications on all energy sources. Here, on-shore wind (11gCO2e/kWh) has a slight edge over nuclear power plants (12g) and off-shore wind (12g).

In this study, hydropower is seen as a rather clean energy source with emissions of only 24g CO2 equivalent (compared to 97g in the first study outlined) and in contrary, the IPCC sees domestic solar PV as much dirtier with 41 grammes (of CO2 equivalent) and large-scale solar farms with 48g.

Biomass is, again, somewhere between clean renewables and traditional fossil fuels. The report calculates its impact at 230gCO2e/kWh.

The findings for fossil fuels reveals the impact of the age of the study. As the IPCC (in 2011) did not consider CCS sources but old-fashioned coal and gas power plants, they emissions skyrocket to 490g (gas) and 820g (coal) of CO2 equivalent.

As we can see, different studies from different years result in different findings. Nevertheless, the order of the greenhouse gas impact is almost identical throughout all studies. Considering that many people see gas power as the best transitional energy source to replace coal and oil and to reach set climate change goals, the biggest question marks lies in the use and power of CCS technologies.

1 Pehl, M., Arvesen, A., Humpenöder, F. et al. Understanding future emissions from low-carbon power systems by integration of life-cycle assessment and integrated energy modelling. Nat Energy 2, 939–945 (2017). https://doi.org/10.1038/s41560-017-0032-9